std::uninitialized_move_n() algorithm

since C++17

// (1)
template< class InputIt, class Size, class NoThrowForwardIt >
std::pair<InputIt, NoThrowForwardIt>
    uninitialized_move_n( InputIt first, Size count, NoThrowForwardIt d_first );

// (2)
template< class ExecutionPolicy, class ForwardIt, class Size, class NoThrowForwardIt >
std::pair<ForwardIt, NoThrowForwardIt>
    uninitialized_move_n( ExecutionPolicy&& policy, ForwardIt first, Size count, NoThrowForwardIt d_first );

• (1) Moves count elements from a range beginning at first to an uninitialized memory area beginning at d_first as if by:

  for (; n > 0; ++d_first, (void) ++first, --n)
      ::new (static_cast<void*>(std::addressof(*d_first)))
          typename std::iterator_traits<NoThrowForwardIt>::value_type(std::move(*first));
  caution

  If an exception is thrown during the initialization, some objects in first + [0, n) are left in a valid but unspecified state, and the objects already constructed are destroyed in an unspecified order.

  Undefined Behaviour

  If d_first + [0, n) overlaps with first + [0, n), the behavior is undefined. (since C++20)

• (2) Same as (1), but executed according to policy.

Overload Resolution

These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (since C++20) is true.

Parameters

first	The beginning of the range of the elements to move.
d_first	The beginning of the destination range.
count	Number of elements to move.
policy	The execution policy to use. See execution policy for details.

Type requirements

InputIt	LegacyInputIterator
ForwardIt NoThrowForwardIt	LegacyForwardIterator

No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions.

Return value

A pair whose first element is an iterator to the element past the last element moved in the source range, and whose second element is an iterator to the element past the last element moved in the destination range.

Complexity

Linear in count.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For none other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

uninitialized_move_n(1)

template<class InputIt, class Size, class NoThrowForwardIt>
std::pair<InputIt, NoThrowForwardIt>
    uninitialized_move_n(InputIt first, Size count, NoThrowForwardIt d_first)
{
    using Value = typename std::iterator_traits<NoThrowForwardIt>::value_type;
    NoThrowForwardIt current = d_first;
    try
    {
        for (; count > 0; ++first, (void) ++current, --count)
            ::new (static_cast<void*>(std::addressof(*current))) Value(std::move(*first));
    }
    catch (...)
    {
        std::destroy(d_first, current);
        throw;
    }
    return {first, current};
}

Examples

Main.cpp

#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <memory>
#include <string>

void print(auto rem, auto first, auto last)
{
    for (std::cout << rem; first != last; ++first)
        std::cout << std::quoted(*first) << ' ';
    std::cout << '\n';
}

int main()
{
    std::string in[]{"One", "Definition", "Rule"};
    print("initially, in: ", std::begin(in), std::end(in));

    if (
        constexpr auto sz = std::size(in);
        void* out = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz))
    {
        try
        {
            auto first{static_cast<std::string*>(out)};
            auto last{first + sz};
            std::uninitialized_move_n(std::begin(in), sz, first);

            print("after move, in: ", std::begin(in), std::end(in));
            print("after move, out: ", first, last);

            std::destroy(first, last);
        }
        catch (...)
        {
            std::cout << "Exception!\n";
        }
        std::free(out);
    }
}

Possible output

initially, in: "One" "Definition" "Rule" 
after move, in: "" "" "" 
after move, out: "One" "Definition" "Rule"